1. Field of the Invention
The present invention relates to a method of manufacturing a nitride-based semiconductor light emitting diode (LED), which can implement a low operation voltage and enhance internal quantum efficiency.
2. Description of the Related Art
In general, a nitride-based semiconductor is such a material that has a relatively high energy band gap (in the case of GaN semiconductor, about 3.4 eV), and is positively adopted in an optical device for generating green or blue short-wavelength light. As for the nitride semiconductor, a material having a compositional formula of AlxInyGa(1-x-y)N (herein, 0≦x≦1, 0≦y≦1, and 0≦x+y≦1) is widely used.
However, since such a nitride-based semiconductor has a relatively large energy band-gap, there are difficulties in forming the ohmic contact with an electrode. Particularly, since a p-type nitride semiconductor layer has a larger energy band-gap, contact resistance increases in the contact portion with a positive(p-) electrode. Such an increase causes an operational voltage of the device to increase, thereby increasing the heating value.
Therefore, there is a demand for a method which enhances electric conductivity of a nitride semiconductor layer of a nitride-based semiconductor LED to reduce an operation voltage and to improve an output, and increases internal quantum efficiency to increase light efficiency.
Now, referring to FIG. 1, a conventional nitride-based semiconductor LED will be described.
FIG. 1 is a cross-sectional view of a conventional nitride-based semiconductor LED.
As shown in FIG. 1, the conventional nitride-based semiconductor LED includes a sapphire substrate 110, a GaN buffer layer (not shown), an n-type nitride semiconductor layer 120, an active layer 130, and a p-type nitride semiconductor layer 140, which are sequentially grown on the substrate 10. Portions of the p-type nitride semiconductor layer 140 and the GaN/InGaN active layer 130 are removed by mesa-etching, so that a portion of the upper surface of the n-type nitride semiconductor layer 120 is exposed.
On the exposed n-type nitride semiconductor layer 120, a negative electrode (n-electrode) 150 is formed of Cr/Au. On the p-type nitride semiconductor layer 140, a positive electrode (p-electrode) 160 is formed of Cr/Au.
The n-type nitride semiconductor layer 120 and the p-type nitride semiconductor layer 140 have a large energy band-gap. Therefore, if the n-type nitride semiconductor layer 120 and the p-type nitride semiconductor layer 140 respectively come in contact with the n-electrode 160 and the p-electrode 160, the contact resistance increases. Such an increase causes an operational voltage of the diode to increase, thereby increasing the heating value.
To solve such a problem, when the n-type nitride semiconductor layer 120 and the p-type nitride semiconductor layer 140 are grown, an amount of doping element in the n-type nitride semiconductor layer 120 and the p-type nitride semiconductor layer 140 is increased, thereby enhancing electric conductivity of the n-type nitride semiconductor layer 120 and the p-type nitride semiconductor layer 140.
Meanwhile, a nitride semiconductor layer with high electric conductivity can be obtained when a ratio at which doping elements injected with high concentration are activated as donors or acceptors is high.
When the n-type nitride semiconductor layer 120 is grown, n-type Si elements serving as doping elements are used as dopants, and most of injected Si elements are activated so that electron concentration of more than 1018 cm−3 can be implemented. However, when the p-type nitride semiconductor layer 140 is grown, p-type Mg elements serving as doping elements are used as dopants like the n-type nitride semiconductor layer 120, but a layer quality is bad and hole concentration is low because of the following three main factors. Therefore, it is difficult to implement a p-type nitride semiconductor layer with high concentration.
The first factor is the bonding of Mg and H within the p-type nitride semiconductor layer 140. More specifically, the p-type nitride semiconductor layer 140 according to the related art is grown by Metal Organic Chemical Vapor Deposition (MOCVD) using Hydrazine-based nitrogen precursor and nitrogen carrier gas. However, most of Mg injected as a doping element when the p-type nitride semiconductor layer 140 is grown is bonded to H and thus is not activated. Further, to solve the above-described problem, the bonding of Mg and H is broken through a heat-treatment process such that a ratio of Mg to be activated increases. However, the activation ratio is still low. Therefore, to reduce resistance of a nitride-based semiconductor LED, an excessive amount of Mg which is ten or hundred times larger than desired hole concentration should be injected when the p-type nitride semiconductor layer is grown. However, the GaN bonding of the p-type nitride semiconductor layer 140 has a different structure from the MgN bonding. Therefore, when an excessive amount of Mg is injected, the layer quality of the p-type nitride semiconductor layer 140 is significantly degraded.
The second factor is the growth temperature of the p-type nitride semiconductor layer 140. In the conventional nitride-based semiconductor LED, the n-type nitride semiconductor layer 120, the active layer 130, and the p-type nitride semiconductor layer 140 are sequentially formed on the substrate 110. Among them, the active layer 130 has a multi-quantum well structure including an InGaN layer. Since InN binding energy is weak, the InGaN layer is grown at a lower temperature by about 200° C. than a GaN layer. In this case, the p-type nitride semiconductor layer 140 is grown at a lower temperature than the n-type nitride semiconductor layer 120, in order to minimize a thermal damage of the active layer 130 which is grown at a relatively low temperature. However, as the growth temperature is low, a mobility of atoms at the grown surface is reduced. As a result, the atoms do not move to the stabilized surface because of short resident time, thereby further degrading a layer quality of the p-type nitride semiconductor layer 140.
The third factor is the generation of N (nitrogen) vacancy. When the p-type nitride semiconductor layer 140 is formed through the MOCVD, an N-vacancy defect when GaN is grown occur in many places, because a decomposition ratio of NH3 gas serving as a supply source of N is extremely low. Since the N-vacancy defect is an n-type defect, an n-type nitride semiconductor is grown even when GaN is grown. Therefore, since such an n-type defect occurs at the same time when the p-type nitride semiconductor layer 140 is grown, hole concentration is further reduced due to a compensation effect.